FindThreads: Honor THREADS_PREFER_PTHREAD_FLAG when pthread is found in libc
The -pthread flag tells the compiler/linker to link to additional libraries needed for thread support (e.g. libatomic on riscv64). The flag therefore should be used if requested using THREADS_PREFER_PTHREAD_FLAG also when the pthread functions are found in libc.
Topic-rename: FindThreads-libc-pthread-flag
Edited by Brad King